1. Field of the Invention
This invention relates generally to a process for removing inorganic, organic, and microbiological contaminants from fluids. In its more particular aspects, the invention relates to the field of such devices that use membranes as one stage of a fluid treatment process. More particularly, a process for removing soluble and insoluble inorganic, organic, and microbiological contaminants from a fluid stream employing a pretreatment module, a post-treatment module, a recycle stream module or any combination thereof, and a membrane module, is provided. The process provided reduces the problems associated with membrane fouling and increases contaminant removal capacity
2. Description of Related Art
Purification of Water:
Purification or filtration of water or other aqueous solutions is necessary for many applications, from the provision of safe or potable drinking water to biotechnology applications including fermentation processing and separation of components from biological fluids, and to industrial processes that require waste stream treatment and recovery of process chemicals. Similarly, the removal of contaminants from fluids used in medical procedures and semiconductor fabrication processes, where ultrapurified fluids are required, and in environments where the fluids will be recirculated, such as aircraft or spacecraft, is also an important application for filtration and fluid treatment materials. In recent years, the need for water filtration and purification in the home has become more recognized, and the competing concerns of energy efficiency and residential fluid quality have lead to numerous filtration products, that purport to remove small particles, allergens, microorganisms, intentionally introduced biotoxins, pesticides, and toxic metals such as lead, mercury, and arsenic.
There are many well-known methods currently used for water purification, such as reverse osmosis, distillation, ion-exchange, chemical adsorption, coagulation, and filtering or retention. Particle filtration may be completed through the use of membranes or layers of granular materials. Other fluid purification techniques involve chemical introduction which alters the state or chemical identity of the contaminant. Examples of chemical additives include oxidizing agents, flocculating agents, and precipitation agents.
In many fluid purification applications a combination of techniques are required in order to completely purify fluids, such as water. Combinations of technologies may be implemented by combining functions in a single device or using several different devices and technologies in series where each performs a distinct function. Examples of this practice include the use of mixed ion-exchange resins that remove both negative and positively charged chemical species and oxidation/filtration methods where oxidizers are used to generate particulate matter that may be subsequently filtered.
The use of membrane materials in fluid treatment operations has become a mainstay of the field. Membrane filters are currently commercially available in single sheets or in multi-sheet formats which are spirally wound. Membranes can be generated with a range of pore sizes and pore size distributions, chemical surface functionalities, physical properties, and size. In combination, these properties determine the adequacy of the membrane for the application. Membranes are categorized with respect to pore size and include but are not limited to microfiltration membranes, ultrafiltration membranes, nanopore membranes, and reverse osmosis membranes. Microfiltration membranes are commonly used to remove insoluble particulate matter from fluid streams while reverse osmosis membranes and nanopore membranes are used to separate water from fluids which contain both water and dissolved organic and inorganic contaminants. These membranes can often be used to generate potable water from sea water.
It is well understood that as the pore size of the membrane decreases the problems with surface fouling increase. As example reverse osmosis membranes are subject to fouling from precipitated salts, adsorbed particulate matter including microorganisms, and from chemical degradation. Surface fouling results in dramatically decreased performance of the membrane in both contaminant removal and fluid passage through the membrane (permeate).
As a result, membranes with very small pore sizes, including nanopore and reverse osmosis membranes require prefilters which reduce contact with large insoluble particulate matter, careful control of raw water conditions, and in many cases repetitive cleaning operations.
Prefiltration technologies are known to prolong the lifetime of reverse osmosis membranes. Depending upon the membrane type both particulates and oxidizing chemicals such as water treatment introduced chlorine must be removed in a preliminary treatment step. When only particulate prefiltration is required it is common to use a string-wound prefilter. When both particulate and chlorine reduction is required activated carbon prefilters are used. Activated carbon filters are commercially available in both granular and block (molded or extruded) formats.
Membrane efficiency and contaminant rejection levels are however also related to the chemical and physical identity of the contaminant. Uncharged (neutral) and weakly charged contaminants are often poorly rejected by ultra, nanopore, and reverse osmosis membranes. Important examples of this situation include the low rejection levels of trivalent arsenic relative to pentavalent arsenic, and the lack of rejection of many chlorinated compounds originating from the water treatment disinfection process.
There is significant prior art in the field of water treatment systems employing reverse osmosis membranes. Specifically, there is significant art associated with the manufacture of membranes, pH adjustment of “raw” fluids through chemical injection before membrane introduction, the cleaning of membranes, the back flushing of membranes, and the design of automated and semi-automated treatment systems employing many variations of these procedures. Unfortunately, these modifications to the basic concept of pressurizing fluid against the membrane and collecting treated fluid from the low-pressure side of the membrane adds significant technical complexity and cost of operation, and decreases the safety associated with the process. As a result many water treatment systems comprised of nanopore, ultra, or reverse osmosis membranes coupled with advanced treatment technologies included for the purposes of extending membrane life and improving contaminant rejection are not suited for residential point-of-use or point-of-entry applications, and increasing are not suited for many industrial applications.
Accordingly one object of this invention is to provide a nanopore, ultra, and/or reverse osmosis membrane based water treatment system which employs inexpensive, safe, and reliable membrane pretreatment and post-treatment fluid conditioning. The process of the invention also serves to protect the membrane from particulate and chemical contaminants, improve the rejection level of some contaminants, and reduce the concentration of contaminants that are rejected poorly by the membrane, all of which are objects of the invention.